1. Field of the Invention
The present invention relates to a semiconductor laser device including a light receiving element for receiving a monitoring laser beam.
2. Description of the Related Art
The structure of a conventional semiconductor laser device will be explained below with reference to FIGS. 1A, 1B, and 1C. FIG. 1A is a plan view showing the structure of the conventional semiconductor laser device. FIG. 1B is a sectional view taken along a line 1B-1B in the plan view. FIG. 1C is a front view showing the semiconductor laser device from the side of a laser beam emitting surface.
As shown in FIGS. 1A to 1C, a semiconductor laser element mount bed (to be referred to as a semiconductor laser element hereinafter) 101 is mounted on a silicon substrate 102. A light receiving element 103 is formed in the silicon substrate 102. The silicon substrate 102 is mounted on a lead frame 104 having a mold.
Lead terminals 105 are arranged around the lead frame 104. Bonding wires 106 are formed between these lead frames 105, the silicon substrate 102, and the light receiving element 103, and between the lead frame 104 and the semiconductor laser element 101. In addition, a package 107 is formed on the lead frame 104 so as to cover the semiconductor laser element 101, the silicon substrate 102, the light receiving element 103, and the bonding wires 106.
In the semiconductor laser device, the characteristics of the semiconductor laser element 101 rapidly deteriorate with time, if stress is applied to the semiconductor laser element 101 by thermal history during assembly or generation of heat during laser oscillation. This shortens the life of the semiconductor laser element 101. To prevent this, the semiconductor laser element 101 is not directly mounted on the lead frame 104, but is mounted on a silicon substrate 102 having a relatively close linear expansion coefficient.
One end face 101A of the semiconductor laser element 101 emits a principal laser beam L1. Another end face 101B opposite to the end face 101A emits a monitoring laser beam L2. The monitoring laser beam L2 is emitted backward from the other end face 101B, and enters the light receiving element 103. The monitoring laser beam L2 is photoelectrically converted by the light receiving element 103 and detected as an electric current.
The structure of the semiconductor laser device shown in FIG. 1B makes the principal emission direction of the monitoring laser beam L2 parallel to the light receiving surface of the light receiving element 103. The principal emission direction is one of the emission directions of the monitoring laser beam L2, in which the optical intensity of the emitted laser beam L2 is a maximum.
As described above, when the light receiving surface of the light receiving element 103 is parallel to the principal emission direction of the monitoring laser beam L2, most of the monitoring laser beam L2 does not enter the light receiving element 103, but is absorbed as useless light by the package 107 and the lead frame 104, or scattered. However, since the monitoring laser beam L2 is emitted with a certain angle, the light receiving element 103 picks up a portion of the monitoring laser beam L2 outside the principal emission direction, and converts the light into a monitoring electric current. In the conventional semiconductor laser device, therefore, the light receiving ratio at which the light receiving element 103 receives the monitoring laser beam L2 is very low. To minimize the inconvenience, the light receiving element 103 is usually positioned as close as possible to the monitoring laser beam emission point. This largely limits the position of the semiconductor laser element 101.
Also, scattered light which does not enter the light receiving element 103 but is reflected from the package 107 and the lead frame 104 enters the semiconductor laser element 101. The scattered light functions as noise to disturb laser oscillation.